Fluid depletion during the perioperative period is associated with poorer outcome. Non-invasive measurement of total body water by bioimpedance may enable preoperative fluid depletion and its influence on perioperative outcome to be assessed.Weight and foot bioimpedance were recorded under standardized conditions in patients undergoing bowel preparation (n=43) or day surgery (n=44). Fifteen volunteers also followed standard nil-by-mouth instructions on two separate occasions to assess the variabilities of weight and bioimpedance over time.Body weight fell by 1.27 kg (95% CI 1.03-1.50 kg; P<0.0001) and foot bioimpedance increased by 51 ohm after bowel preparation (95% CI 36-66; P<0.0001). Weight change after the nil-by-mouth period in day-surgery patients (mean -0.22 kg, 95% CI -0.05 to -0.47 kg; P=0.07) correlated (r=-0.46; P=0.005) with an increase in bioimpedance (16 ohms, 95% CI 5-27 ohms; P=0.01). No difference between two separate bioimpedance measurements was seen in the volunteer group.Further work is warranted to determine if bioimpedance changes may serve as a useful indicator of perioperative fluid depletion.